decapods off Balearic Islands (western Mediterranean):
from single to multi-species approach Beatriz Guijarro
Ph.D. Thesis
P OPULATION DYNAMICS AND ASSESSMENT OF EXPLOITED DEEP WATER DECAPODS OFF B ALEARIC
I SLANDS ( WESTERN M EDITERRANEAN ): FROM SINGLE TO MULTI - SPECIES APPROACH
Beatriz Guijarro González
© B. Guijarro
© De la portada: B. Guijarro
© De les fotografies: IEO-COB Correcció lingüística:
C. Stonehouse: capítols 3, 4 J. Rogerson: capítol 5 C. Rodgers: capítol 7 Adreça actual:
Instituto Español de Oceanografía Centre Oceanogràfic de les Balears Moll de Ponent s/n
07015 Palma (Illes Balears) [email protected]
TESIDOCTORAL
POPULATION DYNAMICS AND ASSESSMENT OF EXPLOITED DEEP WATER DECAPODS OFF BALEARIC ISLANDS (WESTERN MEDITERRANEAN): FROM
SINGLE TO MULTI-SPECIES APPROACH
Memòria presentada per Beatriz Guijarro González per optar al títol de doctora del Programa de Doctorat d‟Ecologia Marina de la Universitat de les Illes Balears, sota la direcció del Dr. Enric Massutí Sureda i del Dr. Joan Moranta Mesquida, i la ponència del Dr. Rafael Bosch Zaragoza.
Beatriz Guijarro González Palma, Maig 2012
ViP El director,
Dr. Enric Massutí Sureda IEO-Centre Oceanogràfic de les Balears
ViP El director,
Dr. Joan Moranta Mesquida IEO-Centre Oceanogràfic de les Balears
ViP El ponent,
Dr. Rafael Bosch Zaragoza UIB-Universitat de les Illes Balears
“Entre l‟arena i el mar no hi ha aroma més fina que el perfum de saladina que a ta pell dus amarat”
(„Entre l‟arena i el mar‟, Xaloc Música)
“A la mar se cria el peix i a la figuera les figues en els teus llavis besades i a les meves mans carícies”
(Fandango „La Malaguenya‟, Música Nostra)
“Amb sa xarxa hi pots agafar peixos en molta quantitat:
alerta que si ve al cas no ens quedem sense cap.
[...]
Dins ella hi pots trobar crancs, eriçons i copinyes, peixos d'espècies molt fines, sirenes i cavalls de mar”
(„La Mar‟, Pla Forana)
VII
Table of contents
Acknowledgements/Agraïments/Agradecimientos IX
Funding XIII
Aim of the thesis XV
Structure of the thesis XVII
Summary XIX
Resum XXIII
Resumen XXVII
Chapter 1. Introduction 31
1.1. Introduction 33
1.2. The study area 35
1.3. The bottom trawl fishery off the Balearic Islands 41
1.4. The deep water crustacean fishery 45
1.5. Deep water crustaceans 49
1.5.1. Aristeus antennatus 52
1.5.2. Pandalidae and Pasiphaeidae 54
1.5.3. Parapenaeus longirostris 57
1.5.4. Geryon longipes 59
1.5.5. Nephrops norvegicus 61
Chapter 2. Data source and sampling strategies 63
2.1. Introduction 65
2.2. Fishery-independent data 65
2.2.1. Seasonal experimental surveys 65
2.2.2. Annual experimental bottom trawl surveys 72
2.3. Fishery-dependent data 75
2.3.1. Daily sale bills 75
2.3.2. Seasonal fleet monitoring 76
2.4. Selectivity pilot surveys 78
Chapter 3. Parapenaeus longirostris 83
Abstract 85
3.1. Introduction 86
3.2. Materials and methods 87
3.2.1. Data source 87
3.2.2. Data analysis 88
3.3. Results 90
3.4. Discussion 100
VIII
Chapter 4. Pandalidae 105
Abstract 106
4.1. Introduction 107
4.2. Materials and methods 110
4.2.1. Data source 110
4.2.2. Data analysis 110
4.3. Results 112
4.4. Discussion 123
Chapter 5. Aristeus antennatus 133
Abstract 135
5.1. Introduction 136
5.2. Materials and methods 137
5.2.1. Data source 137
5.2.2. Data analysis 138
5.3. Results 140
5.4. Discussion 152
Chapter 6. Assessment 161
Abstract 163
6.1. Introduction 164
6.2. Materials and methods 166
6.2.1. Data source 166
6.2.2. Data analysis 166
6.3. Results 170
6.4. Discussion 178
Chapter 7. Selectivity 185
Abstract 187
7.1. Introduction 188
7.2. Materials and methods 189
7.2.1. Data source 189
7.2.2. Data analysis 190
6.3. Results 191
6.4. Discussion 198
Chapter 8. Conclusions 211
References 225
IX
Acknowledgements/Agraïments/Agradecimientos
És increïble haver arribat fins aquí.
Ha passat tant de temps, tantes coses i tanta gent des de la primera vegada que se me va passar pel cap dedicar-me a això, que crec que serà impossible mencionar a tothom. Ni tan sols sé què he de dir. Però com vaig llegir una vegada a qualque lloc:
When you can’t think of what to say... just say “thank you”.
Thank you... Gràcies... Gracias... Ευχαριστό...
Gracias en primer lugar a mis padres, por comprenderme y apoyarme, incluso sin entender muy bien qué es esto a lo que me dedico. Gracias por haberme dejado tomar mis decisiones, por enseñarme a ser quien soy y por educarme en el trabajo duro.
Gracias a Sara, por ser mi amiga además de mi hermana. Por escuchar mis ralladas mentales y por obligarme a relativizar las cosas.
Gracias al Dr. Luis Gállego, por ser el responsable de que me metiera en este mundo de la ciencia, en este mundo del mar. Si hay que ponerle un inicio a todo esto, sería aquel día que fui por su despacho a ofrecerme a colaborar en el departamento de Zoología.
Gracias a Luis tuve mi primer contacto con el Instituto Español de Oceanografía (IEO), hace más de diez años. Y así empezó todo.
Gracias a todos mis amigos, a los que he dejado más de una vez tirados cuando me han propuesto algún plan, por quedarme en casa con la tesis. Gracias por entender lo importante que ha sido esta tesis para mí. Gracias a mis compis de carrera, Montse, Espe, Judhit, Samuel, Ádamo, Malén. Qué buenos años, aquellos. Gracias a las damas del feisbuk por las charlas digitales en nuestros muros virtuales. Gracias a mis amiguitos más frikis, Marga, Javi, Julià, Obdúlia, Cristina y familia, Miguel Ángel y Mª del Mar, por alegrarme las mañanas, sacarme de casa incluso cuando no quería y decirme cosas como “déjate de tonterías y ¡acaba la tesis de una vez!”. Gracias a Jose, por ser un corrector implacable. Gràcies també als companys de Rafeubetx, tant els músics (Pep Lluís, Miquel, Maria, Joana, Imma i Magdalena) com els balladors (Marta, Javi, Miquel, Rosa, Enrique, Marie Pierre, Rosalia i Sandra). Gràcies per aguantar les meves absències, per les hores de música i ball que hem compartit i per ajudar-me a oblidar de tant en tant del món de la ciència i gaudir dels balls i les cançons. Cap festa sense xeremies!
X
Gràcies a tots els companys de l’IEO, als que han compartit despatx amb jo, feina, berenars, festes, dinars, campanyes, viatges, reunions... És increïble la quantitat de persones que han passat per la meva vida durant aquest més de 10 anys a l’IEO. Tots i cadascun de vosaltres heu significat qualque cosa per jo. A Xisco –ets el millor!-, Toni i Maria, de l’equip de demersals. Gracias Manolo por ser un gran compi de la gran aventura que fue el proyecto IDEA. Gracias a Ana, mamá pato, por ser compañera y amiga. Gràcies també a tots els que heu respost la gran pregunta dels darrers temps
“¿Con qué programa editaste tu tesis?”.
Gràcies als patrons, mariners i armadors amb els que he coincidit durant aquest anys en vaixells comercials i oceanogràfics. Gràcies al patrons, tripulants i armadors de les barques d’arrossegament Arnau i Marc, Es Llevant, Antonia Munar Segunda, Villa de Sóller II, Josep del Paraguay y Marruza, per deixar-me formar part de la vostra vida a la mar de tant en tant. Gràcies molt especialment a en Joan Jesús, patró del Punta del Vent, i a tota la tripulació de l’antic Moralti Nou (Damià, Oscar, Manuel) amb els que vàrem fer les campanyes del projecte IDEA i dels projectes de selectivitat. Gràcies per fer-me sentir com a casa amb vosaltres. Joan, ja saps lo molt que t’estimo!
Thank you to all the people from the ECOSUMMER project (Marie Curie Research Training Network, EU Sixth Framework Programme). Thank you especially to its co- ordinator, Graham Pierce, and to all those who selected my project. Thank you for the time I spent in Crete. Working full time in my PhD during four months was the biggest present ever. Thank you to all the other ECOSUMMER students and people I met in their courses, (Sarah, Ruth, Evina, Consuelo, Dora, Christian, Katie, Iñigo, Patrícia, Ricardo, Gema, Sonia, Fiona, Sabine, Rebeca, Vagelis and Jennifer), who kindly adopted me in the group. It was a pleasure to meet you all, to share with you courses, meetings and worries for the future. It is amazing how much spread we all are, but it is also wonderful knowing that you all are still there. Thank you to Vasilis Valavanis, for giving me the first steps in Crete and helping me with many things. Thank you to George Tserpes, my supervisor there, for his help and friendship. Gracias también a Nota, era un verdadero placer utilizar mi idioma materno contigo estando tan lejos de casa. Ευχαριστό πολί! Thank you Sarah for your help and friendship. Thank you to Vagelis, Matina and Evina, my colleagues in the office in the HCMR. Thank you to all the people at Zorbas Island (Eric, Nienke, Helen), a great place for a beer during those warm Cretan nights. And thank you especially to my neighbour Miguel. You know
XI
Crete wouldn’t have been the same without you! Thank you for the beers in the balcony and the conversations under the stars. It was a pleasure meeting you, as it was meeting us again in Lisbon, Palma and Brussels. Sincerely, I wonder in which city we will meet next!
Gràcies als meus directors de tesi, el Dr. Enric Massutí i el Dr. Joan Moranta. Gràcies Joan per tenir sempre una paraula amable i encoratjadora, per orientar-me en el sempre difícil món de l’estadística i per fer-me plantejar-me qüestions que probablement a jo mateixa ni se m’haguessin passat pel cap. Gràcies Enric per confiar en jo des de fa ja tant d’anys. Gràcies per plantejar-me reptes contínuament i gràcies per creure’m capaç d’assolir-los fins i tot quan ni jo mateixa m’ho creia. Gràcies per dir les paraules adequades en els moments adequats, per donar-me la meva primera oportunitat (i moltes més), per haver fet de jo la científica que (pareix que) som i per donar-me canya fins a les últimes conseqüències.
Quant més s’ha aproximat el final d’aquesta tesi, més m’he adonat que això no és el final de res i tampoc no és el començament de tot. És, simplement, un pas més en el camí d’aquesta meravellosa aventura que vivim dia a dia.
I a tots els que la feu possible...
... gràcies!
XIII Funding
Part of the data used in chapters 3, 4 and 5 comes from the IDEA project (Influence of oceanographic structure and dynamics on Demersal populations in the waters of the Balearic Islands; www.ba.ieo.es/idea), funded by the Ministry of Science and Technology of the Spanish Government (Plan Nacional I+D+I REN2002-04535-C02- 01/02/MAR and REN2002-10670/71-E/MAR).
Part of the data used in chapters 3, 4, 5 and 6 comes from the BALAR and MEDITS surveys, carried out in the framework of the MEDER (Evaluación y bases técnicas para la gestión de las pesquerías demersales), DEMO (Evaluación de recursos y bases técnicas par al agestión de pesquerías demersales en el Mediterráneo occidental), EVADEM (Evaluación de pesquerías demersales en el Mediterráneo), EVADEMED (Evaluación de recursos demersales del mediterráneo) and BADEMECO (Estudio integrado de los ecosistemas demersales explotados en las Islas Baleares (Mediterráneo occidental) y bases científico-técnicas para un enfoque ecosistémico en la gestión de pesquerías) projects, from the Instituto Español de Oceanografía, and partially funded from 2007 by the European Union (EU, Data Collection Regulation and Data Collection Framework).
Chapter 6 is a result of a research training stage in the Hellenic Centre for Marine Research (Ελληνικό Κέντρο Θαλασσίων Ερευνών) in Crete (Greece), which was possible thanks to a fellowship from the Marie Curie Research Training Network under the EU Sixth Framework Programme, in the ECOSUMMER project (Ecosystem approach to Sustainable Management of the Marine Environment and its Living Resources, www.abdn.ac.uk/ecosummer).
Chapter 7 is a result of two experimental pilot projects (RAI-AP-22-2001 and RAI-AP- 6-2002) carried out under the proposal of an ownership firm (Albertí Morey, S.A.) and funded by the Secretary of Fisheries (Ministry of Agriculture, Fisheries and Food).
XV Aim of the thesis
Deep water decapod crustaceans form a considerable fraction of the megafaunal biomass in the upper and middle slope in the western Mediterranean. They are also a very important component of the catches of the commercial fishery, like it is the case in the bottom trawl fishery which operates off the Balearic Islands. In the Balearic Islands, the most important species in terms of biomass is Aristeus antennatus, followed by a mixed category of shrimps (including Pandalidae and Pasiphaeidae), Parapenaeus longirostris, Geryon longipes and Nephrops norvegicus. Economically, A. antennatus is also the most important species, followed by N. norvegicus, the mixed category and P.
longirostris.
The knowledge on deep water decapod crustaceans is mostly centred on single species and generally dependent on their economic importance in the commercial fisheries. Thus, studies aimed at A. antennatus are abundant in many areas, including the Balearic Islands, while those related to other species such as P. longirostris and Pandalidae are scarcer and, in the case of the Balearic Islands, practically absent. The main objective of this thesis is to improve the knowledge of deep water decapod crustaceans off the Balearic Islands, by studying not only the main target commercial species, but also their communities, through a more ecosystemic approach.
Although some studies have already focussed on the population dynamics and biology of the main commercial species, they are analysed here from a more detailed approach, taking into account the different environmental regimes found at two nearby locations from the Balearic Islands. Thus, population dynamics, biology and condition of P. longirostris, Pandalidae and A. antennatus are analysed separately for both locations. For some species, the possible relation with environmental factors has been discussed in different studies, but joint analysis using both biological and environmental data are scarce. This approach, which is applied here to P. longirostris, five Pandalidae and A. antennatus, can be considered as an advance of previous studies in the area.
The above mentioned ecosystemic approach involves studying not only the commercial species, but the entire epi-benthic community and has been applied here to characterise the species assemblages in the slope trawl fishing grounds. The assessment of the impact of fishing in each of these assemblages has been performed using several
XVI
ecological indicators, which provide wide and complementary information, and which are summarized here using the Traffic Light method. This approach, firstly proposed for being used in data poor situations but useful to all kinds of stocks, appears to be more precautionary than traditional stock assessment methods. Both ecological indicators and the Traffic Light method are also used to assess the state of the main target species of the identified assemblages, N. norvegicus and A. antennatus, which have been previously assessed using more traditional stock assessment models, such as virtual population analysis.
Finally, the potential effects of new management measures to improve the selectivity and to reduce the fishing impact are analysed. This analysis is performed by comparing catch composition, commercial yields, retention efficiency, discards and size selectivity parameters using two different mesh shapes, the 40 mm diamond mesh codend in force until 2010 and the 40 mm square mesh codend, in force since then.
XVII Structure of the thesis
This thesis is a collection of scientific papers, preceded by an Introduction (Chapter 1), in which the characteristics of the study area, the description of the bottom trawl fishery operating in the deep water fishing grounds off the Balearic Islands, as well as a short explanation of geographic distribution and population structure of the main decapod crustaceans landed are presented.
Chapter 2 describes all the data sources used to develop the subsequent chapters.
The population dynamics of the main deep water decapods crustaceans off the Balearic Islands, taking into account the influence of the environmental conditions, are analysed in Chapters 3, 4 and 5. The species were selected according to their biomass landed by the bottom trawl fleet and presented in this thesis according to their bathymetric distribution, from shallow to deep waters: Parapenaeus longirostris (Chapter 3), Pandalidae (Chapter 4) and Aristeus antennatus (Chapter 5). Each of these chapters can be read separately, because they have their own Summary and Keywords, an Introduction, as well as Material and Methods, Results and Discussion.
The species assemblages in the deep water trawl fishing grounds as well as the assessment of the impact of fishing on these assemblages are analysed in Chapter 6, by means of ecological indicators, both at species and at community level.
The potential effect of recently introduced new management measures to improve the selectivity and to reduce the impact of the fishery on the ecosystem in the deep water trawl fishing grounds is analysed in Chapter 7.
Finally, Chapter 8 summarizes the main Conclusions.
XIX Summary
In the western Mediterranean, deep water decapod crustaceans form a considerable fraction of the megafaunal biomass in the upper (US, 200-500 m) and middle slope (MS, 500-800 m), being also a very important component of the catches of the commercial fishery. In the Balearic Islands, they represent around 17% in weight and 50% in revenues of the bottom trawl fishery, which operates between 50 and 800 m depth. The most important species in terms of biomass is Aristeus antennatus, followed by a mixed category of shrimps (including Pandalidae and Pasiphaeidae), Parapenaeus longirostris, Geryon longipes and Nephrops norvegicus. Economically, A. antennatus is also the most important species, followed by N. norvegicus, the mixed category and P.
longirostris.
The main objective of this thesis is to analyse the community and population dynamics of the deep water decapod crustaceans off the Balearic Islands. The partial objectives are: (i) to study the short spatial and temporal patterns of the main populations of two nearby locations with different environmental regimes; (ii) to explore relationships between the main species abundances and the environmental conditions; (iii) to characterise the species assemblages in the slope trawl fishing grounds; (iv) to estimate ecological indicators, both at species and at community level, to assess the impact of fishing in these communities, and (v) to analyse the effect of new management measures to improve the selectivity and to reduce the fishing impact.
Data have been obtained from different sampling strategies: (i) seasonal and annual scientific surveys (fishery-independent data), and (ii) directly from the fishing sector (fishery-dependent data). A third type of data was obtained from scientific selectivity pilot studies. During the scientific surveys and the pilot studies, abundance, biomass and length frequency distributions of all the species were collected. Biological information of A. antennatus, P. longirostris and Pandalidae were recorded during the scientific surveys, and during the seasonal surveys different environmental parameters were obtained. Fishery-dependent data came from daily sale bills and seasonal on board sampling monitoring of the bottom trawl fishery. In the pilot studies, information obtained was for landings and discards of two types of codends with different mesh shape (diamond and square) as well as revenues from landings.
XX
Univariate and multivariate techniques have been used to analyse the data, including analysis of variance; cluster, redundancy and similarity percentage analysis;
non-parametric correlations; multiple regressions and general additive models.
Indicators of the state of the main species and the ecosystem were computed for fishery- independent data (annual surveys) and fishery-dependent data (daily sale bills) and summarised by means of the Traffic Lights methodology. Size selectivity for the two codends tested was modelled using the generalized logistic curve and length at first catch was also calculated.
The results show spatial and temporal differences for the species studied between the two locations analysed, in the Algerian (AsB) and Balearic (BsB) sub- basins. These differences are related to higher abundance of P. longirostris and A.
antennatus in the BsB than in the AsB, while the opposite has been found for Pandalidae (except for one species). A better condition has been found for all the species in the BsB than in the AsB. The different environmental conditions between both locations (geomorphological structure, bottom characteristics, hydrographic factors and availability of potential trophic resources) can be on the basis on the differences found, with varied influence among species. In most cases, both reproductive period and condition show a marked seasonality, with spawning concentrated in summer, where the minimum values in the condition has been detected.
The results obtained from the analysis of indicators show different trends for the US and MS assemblages. The state of the US and of its main target species, N.
norvegicus, seem to have improved for the last years, probably related to a fishing effort decrease. However, other factors than fishing effort may affect this species as its worst state is found in the less exploited area. For the MS and its target species, A. antennatus, no clear trends were found for both sources of data (fishery-independent and fishery- dependent). This can be explained by the different species included in each analysis as well as the different temporal sampling periods covered, as in some cases, such as A.
antennatus, the high seasonal variability of its population dynamics is not reflected in the annual surveys (once a year).
When the two codends tested are compared, there is no difference in the catch composition or the yield between the two mesh shapes, although the percentage of total and commercial species discarded with a diamond mesh was higher than with a square
XXI
mesh. At least in the short term, the escapement ratio and the economic loss with the diamond mesh were lower than with square mesh, but economic efficiency was no different between them. For all the main species compared, except one flatfish, size selectivity parameters were lower for the diamond than for the square mesh codend.
Within the context of precautionary management, the introduction of the square mesh in the codend could be an appropriate and plausible measure to improve the state of the resources exploited by the deep water crustacean trawl fishery of the slope off the Balearic Islands and to reduce the impact of the fishery on the ecosystem.
The knowledge derived from the present thesis provide relevant information about how the species traits are mainly affected by water masses, sediment characteristic, trophic resources and fishing, since they influence the bathymetric distribution, abundances, biological parameters and condition of decapod crustaceans at a local scale. To understand how environmental characteristics and fishing impact interact to change the species distribution and the dynamics of the populations and communities is a key point for a better assessment of the resources and for a better application of the Ecosystem-Based Approach Management in the Mediterranean deep water ecosystems.
XXIII Resum
Al Mediterrani occidental, els crustacis decàpodes de profunditat formen una considerable fracció en la biomassa de la megafauna del talús superior (US, 200-500 m) i el talús mitjà (MS, 500-800 m), essent a més una part important de les captures de la pesca comercial. A les Illes Balears, representen aproximadament un 17% en pes i un 50% dels guanys de la pesqueria d’arrossegament de profunditat, que opera entre 50 i 800 m de fondària. L’espècie més important en biomassa és Aristeus antennatus, seguida d’una categoria de gambes mesclades (que inclou Pandalidae i Pasiphaeidae), Parapenaeus longirostris, Geryon longipes y Nephrops norvegicus. Des d’un punt de vista econòmic, A. antennatus és també l’espècie més important, seguida de N.
norvegicus, la categoria de mescla i P. longirostris.
L’objectiu principal d’aquesta tesi és analitzar la comunitat i la dinàmica poblacional dels crustacis decàpodes de profunditat de les Illes Balears. Els objectius parcials són: (i) estudiar els patrons espacials i temporals a petita escala de les principals poblacions de dues zones properes, amb diferents règims ambientals; (ii) explorar les relacions entre l’abundància de les principals espècies i les condicions ambientals; (iii) caracteritzar les associacions d’espècies en els caladors d’arrossegament del talús; (iv) estimar indicadors ecològics, tant a nivell d’espècies com a nivell de comunitat, per avaluar l’impacte de la pesca en aquestes comunitats i (v) analitzar l’efecte de noves mesures de gestió per millorar la selectivitat i reduir l’impacte de la pesca.
Les dades s’han obtingut a partir de diferents estratègies de mostreig: (i) campanyes científiques estacionals i anuals (dades independents de la pesqueria) i (ii) directament del sector pesquer (dades dependents de la pesqueria). Un tercer tipus de dades s’han obtingut mitjançant estudis pilot de selectivitat. Durant les campanyes científiques i els estudis pilot, es va recollir informació de l’abundància, biomassa i distribució de la freqüència de talles de totes les espècies. Durant les campanyes científiques, es recollí informació biològica d’A. antennatus, P. longirostris i Pandalidae i durant les campanyes estacionals s’obtingueren diferents paràmetres ambientals. Les dades independents de la pesqueria foren fulles de venda diàries i mostratges estacionals a bordo de la flota d’arrossegament. En els estudis pilot, es recollí informació pels desembarcaments i els rebutjos, amb els dos tipus de cops de
XXIV
diferent geometria de malla (ròmbica i quadrada), així com els guanys dels desembarcaments.
S’han utilitzat diferents tècniques univariants i multivariants, incloent l’anàlisi de variància; anàlisi cluster, de redundància i el percentatge de similitud; correlacions no paramètriques; regressions múltiples i models additius generalitzats. Se calcularen els indicadors d’estat de les principals espècies i de l’ecosistema per les dades independents de la pesqueria (campanyes anuals) i les dades dependents de la pesqueria (fulles de venda diàries) i es resumiren utilitzant la metodologia dels semàfors. La selectivitat dels dos cops utilitzats es modelà utilitzant la corba logística generalitzada i es calculà la talla de primera captura.
Els resultats mostren diferències espacials i temporals per les espècies estudiades entre les dues zones analitzades, a la sub-conca algerina (AsB) i la balear (BsB).
Aquestes diferències són una major abundància de P. longirostris i A. antennatus a la BsB que a la AsB, mentre que per Pandalidae (excepte per una espècie) s’ha trobat el contrari. Totes les espècies presenten millor condició en la BsB que en la AsB. Les diferents condicions ambientals en ambdues zones (estructura geomorfològica, característiques del fons, factors hidrogràfics i disponibilitat de potencials recursos tròfics) poden ser la base de les diferències trobades, amb distinta influència segons les espècies. A la majoria dels casos, tant el període reproductiu com la condició mostren una marcada variabilitat, amb la fresa concentrada en estiu, quan la condició presenta els valors més baixos.
L’anàlisi d’indicadors mostra diferents tendències per a les associacions del US i MS. L’estat del US i de la seva principal espècie objectiu, N. norvegicus, pareix haver millorat durant els darrers anys, probablement degut amb una disminució de l’esforç pesquer. No obstant, a més de l’esforç pesquer altres factors podrien influir en aquesta espècie, ja que el pitjor estat es troba a la zona menys explotada. Pel MS i la seva espècie objectiu, A. antennatus, no s’ha detectat una tendència clara en les dues fonts de dades (dades independents i dependents de la pesqueria). Això se pot explicar per les diferents espècies incloses en cada anàlisi, així com la diferent cobertura temporal dels mostratges, ja que en alguns casos, com per A. antennatus, la gran variabilitat estacional de la seva dinàmica poblacional no es reflecteix en les campanyes anuals (que es duen a terme una vegada a l’any).
XXV
Quan se comparen els dos cops testats, no s’han trobat diferències en la composició de la captura o en els rendiments entre les dues geometries de malles, malgrat el percentatge total d’espècies rebutjades i d’espècies comercials rebutjades amb malla ròmbica fou major que amb malla quadrada. La proporció d’escapament i la pèrdua econòmica amb la malla ròmbica fou millor que amb malla quadrada, almenys a curt termini, però l’eficiència econòmica no fou diferent entre malles. Per totes les espècies comparades, excepte un peix pla, els paràmetres de selectivitat foren menors amb el cop de malla ròmbica que amb el de malla quadrada. En el context de gestió basada en el principi de precaució, la introducció de la malla quadrada en el cop pot ser una mesura apropiada i plausible per millorar l’estat dels recursos explotats per la pesqueria de crustacis de profunditat del talús de les Illes Balears i per reduir l’impacte de la pesca a l’ecosistema.
Els coneixements derivats d’aquesta tesi proporcionen informació relevant sobre com les característiques de les espècies estan principalment afectades per les masses d’aigua, les característiques dels sediments, els recursos tròfics i la pesca, ja que influeixen en la distribució batimètrica, abundància, paràmetres biològics i condició dels crustacis decàpodes a escala local. Entendre com les característiques ambientals i l’impacte de la pesca interactuen per modificar la distribució de les espècies i la dinàmica poblacional de les poblacions i comunitats és un punt clau per una millor avaluació dels recursos i per una millor aplicació de l’aproximació ecosistèmica a la gestió dels ecosistemes marins de profunditat del Mediterrani.
XXVII Resumen
En el Mediterráneo occidental, los crustáceos decápodos de profundidad forman una considerable fracción en la biomasa de la megafauna en el talud superior (US, 200- 500 m) y el talud medio (MS, 500-800 m), siendo además una parte importante de las capturas de la pesca comercial. En las Islas Baleares, representan aproximadamente un 17% en peso y un 50% de las ganancias de la pesquería de arrastre de profundidad, que opera entre 50 y 800 m de profundidad. La especie más importante en términos de biomasa es Aristeus antennatus, seguida de una categoría de gambas mezcladas (que incluye Pandalidae y Pasiphaeida), Parapenaeus longirostris, Geryon longipes y Nephrops norvegicus. Económicamente, A. antennatus es también la especie más importante, seguida de N. norvegicus, la categoría de mezcla y P. longirostris.
El objetivo principal de esta tesis es analizar la comunidad y la dinámica poblacional de los crustáceos decápodos de profundidad de las Islas Baleares. Los objetivos parciales son: (i) estudiar los patrones espaciales y temporales a pequeña escala de las principales poblaciones de dos zonas cercanas, con diferentes regímenes ambientales; (ii) explorar las relaciones entre la abundancia de las principales especies y las condiciones ambientales; (iii) caracterizar las asociaciones de especies en los caladeros de arrastre del talud; (iv) estimar indicadores ecológicos, tanto a nivel de especies como a nivel de comunidad, para evaluar el impacto de la pesca en estas comunidades y (v) analizar el efecto de nuevas medidas de gestión para mejorar la selectividad y reducir el impacto de la pesca.
Los datos se han obtenido a partir de diferentes estrategias de muestreo: (i) campañas científicas estacionales y anuales (datos independientes de la pesquería) y (ii) directamente del sector pesquero (datos dependientes de la pesquería). Un tercer tipo de datos se han obtenido a partir de estudios pilotos de selectividad. Durante las campañas científicas y los estudios piloto, se recogió información de la abundancia, biomasa y distribución de la frecuencia de tallas de todas las especies. Durante las campañas científicas, se recogió información biológica de A. antennatus, P. longirostris y Pandalidae y, durante las campañas estacionales se obtuvieron diferentes parámetros ambientales. Los datos dependientes de la pesquería fueron hojas de venta diarias y muestreos estacionales a bordo de la flota de arrastre. En los estudios piloto, se recogió
XXVIII
información para los desembarcos y los descartes, con dos tipos de copos de diferente geometría de malla (rómbica y cuadrada), así como ganancias de los desembarcos.
Se han utilizados distintas técnicas univariantes y multivariantes, incluyendo el análisis de variancia; análisis cluster, de redundancia y del porcentaje de similitud;
correlaciones no paramétricas; regresiones múltiples y modelos aditivos generalizados.
Se calcularon los indicadores del estado de las principales especies y del ecosistema para los datos independientes de la pesquería (campañas anuales) y los datos dependientes de la pesquería (hojas de venta diarias) y se resumieron utilizando la metodología de los semáforos. La selectividad de los dos copos utilizados se modeló utilizando la curva logística generaliza y se calculó la talla de primera captura.
Los resultados muestran diferencias espaciales y temporales para las especies estudiadas entre las dos zonas analizadas, en la sub-cuenca argelina (AsB) y la balear (BsB). Estas diferencias son una mayor abundancia de P. longirostris y A. antennatus en la BsB que en la AsB, mientras que para Pandalidae (excepto para una especie) se ha encontrado lo contrario. Todas las especies presentaron mejor condición en la BsB que en la AsB. Las diferentes condiciones ambientales en ambas zonas (estructura geomorfológica, características del fondo, factores hidrográficos y disponibilidad de potenciales recursos tróficos) pueden estar en la base de las diferencias encontradas, con distinta influencia según las especies. En la mayoría de los casos, tanto el período reproductivo como la condición muestran una marcada estacionalidad, con el desove concentrado en verano, cuando la condición presenta los valores más bajos.
El análisis de indicadores muestra diferentes tendencias para las asociaciones del US y MS. El estado del US y de su principal especie objetivo, N. norvegicus, parece haber mejorado durante los últimos años, probablemente relacionado con una disminución del esfuerzo pesquero. Sin embargo, además del esfuerzo pesquero otros factores podrían influir a esta especie ya que el peor estado se encuentra en la zona menos explotada. Para el MS y su especie objetivo, A. antennatus no se ha detectado una tendencia clara en las dos fuentes de datos (datos independientes y dependientes de la pesquería). Esto podría explicarse por las diferentes especies incluidas en cada análisis, así como la diferente cobertura temporal de los muestreos, ya que en algunos casos, como para A. antennatus, la gran variabilidad estacional de su dinámica poblacional no se refleja en las campañas anuales (que se realizan una vez al año).
XXIX
Cuando se comparan los dos copos probados, no se han encontrado diferencias en la composición de la captura o en los rendimientos entre las dos geometrías de mallas, aunque el porcentaje total de especies descartadas y de especies comerciales descartadas con malla rómbica fue mayor que con malla cuadrada. La proporción de escape y la pérdida económica con la malla rómbica fue mejor que con malla cuadrada, al menos a corto plazo, pero la eficiencia económica no fue diferente entre mallas. Para todas las especies comparadas, excepto un pez plano, los parámetros de selectividad fueron menores con el copo de malla rómbica que con el de malla cuadrada. En el contexto de gestión basada en el principio de precaución, la introducción de la malla cuadrada en el copo puede ser una medida apropiada y plausible para mejorar el estado de los recursos explotados por la pesquería de crustáceos de profundidad del talud de las Islas Baleares y para reducir el impacto de la pesca en el ecosistema.
Los conocimientos derivados de esta tesis proporcionan información relevante sobre cómo las características de las especies están principalmente afectadas por las masas de agua, las características de los sedimentos, los recursos tróficos y la pesca, ya que influyen en la distribución batimétrica, abundancia, parámetros biológicos y condición de los crustáceos decápodos a escala local. Entender cómo las características ambientales y el impacto de la pesca interactúan para modificar la distribución de las especies y la dinámica poblacional de las poblaciones y comunidades es un punto clave para una mejor evaluación de los recursos y para una mejor aplicación de la aproximación ecosistémica en la gestión de los ecosistemas marinos de profundidad del Mediterráneo.
C HAPTER 1.
I NTRODUCTION
33
1.1. Introduction
Mediterranean fisheries are highly diverse, as they are composed by atomised fleets, a huge number of landing points spread over more than 40000 km of coasts and multispecies catches, with the absence of large monospecific stocks in comparison to those which inhabit some wide areas of the open oceans (Farrugio et al., 1993; Lleonart and Maynou, 2003). Although they only represent a small proportion of the world production (<2%; Figure 1.1), the mean prices of landings (which are mainly sold fresh) are well above the average prices of world markets. Among them, demersal resources constitute a complex fishery as it is composed of a great number of species (over 100) of fish, crustaceans and molluscs, exploited by many different gears, but being the trawl the main one, both in terms of catch and fleet power (Caddy, 1993; Lleonart and Maynou, 2003).
Figure 1.1. The world marine fisheries production by FAO fishery regions in 2005 (Source:
FAO 2007. Fishtat Plus 2.32. Fisheries and Aquaculture Information and Statistics Service;
Food and Agriculture Department).
34
The management of the Mediterranean fisheries is based on effort control. No total allowable catches (TACs) are implemented (except for some species, such as bluefin tuna and swordfish), nor are other types of adaptive management. Other technical measure, such as minimum landing sizes and minimum mesh sizes, are also implemented but not always enforced. Most of the rules concerning the management of demersal fisheries have been developed for trawling, not only because it is the main gear contributing to demersal catches, but also because it presents low selectivity in comparison with the most important artisanal gears (nets, longlines and traps).
According to Spanish regulations (Real Decreto Nº1440/1999 of 10 September 1999 and Orden APA/79/2006 of 19 January 2006) trawling is carried out in fishing grounds deeper than 50 m and shallower than 1000 m, and the activity is limited to a maximum of 5 days per week and 12 hours per day. Trawling is forbidden on Posidonia oceanica (and other marine phanerogams) beds, coralline and maërl bottoms, which are considered protected habitats. The fishing vessels must have a length between 12 and 24 m and a maximum power not greater than 500 horse power. The gear used is a typical otter trawl. The minimum stretched mesh size in the codend allowed for all Spanish Mediterranean waters was 40 mm until 1st July 2010, when the bottom trawl net was replaced by a square meshed net of 40 mm at the codend or, at the duly justified request of the ship owner, by a diamond meshed net of 50 mm. After 1st February 2012, the diamond meshed net of 50 mm can only be used if its acknowledged size selectivity is equivalent to or higher than that of square meshed nets of 40 mm at the codend (Council Regulation, EC Nº1967/2006 of 21 December 2006 and Regulation EU No 1343/2011 of the European Parliament and of the Council of 13 December 2011).
Fishing exploitation in general and trawling in particular has a number of effects, not only because it directly reduces the abundance of commercial stocks, but also because it indirectly affects the by-catch species, by increasing their mortality, and the ecosystem, by disturbing marine habitats. The direct effects of towed gears such as bottom trawling include the scraping, scouring and resuspension of substratum, causing physical impacts on benthic organism an on the seabed (Jennings and Kaiser, 1998;
Goñi, 1998). Fishing exploitation directly reduces the abundance, biomass and size of both target and by-catch species but it also has many indirect implications at community level (e.g. Dayton et al., 1995; Haedrich and Barnes, 1997; Goñi, 1998; Philippart, 1998; Kaiser et al., 1999; Gislason et al., 2000; Kaiser and De Groot, 2000), as it
35
removes potential preys of other species or predators that would otherwise control prey populations, it changes the density of some species which may affect competitive interactions and it provides to scavenging species with food. Some of these effects come from the already mentioned low selectivity of trawling gears, which is particularly relevant in fisheries mainly sustained by the youngest age groups, as it is the case of Mediterranean trawl fisheries (Caddy, 1993; Lleonart, 1999). This, together with the large percentage of catches that comes from species of low or null commercial values (Carbonell et al., 1998; Moranta et al., 2000; Sánchez et al., 2004) and/or undersized individuals of target species (Oliver, 1991), provokes a useless high mortality as a significant part of the catch is discarded.
Although the effects of fishing on marine communities and ecosystems have been well known for long (Jennings and Kaiser, 1998), the scientific basis for the management of fisheries was founded in the study of exploited fish populations, using the target species of the commercial fleet as a primary unit and hence being the monospecific approaches the rule in assessment. In recent decades, there has been a progressive change from the traditional approach of fishery assessment and management, which considers populations as independent and auto-sustainable, to an ecosystemic approach, which considers the ecosystem as an assessment and management unit and takes into account the complexity of the ecosystems, their natural variations and the factors that control these changes, as well as the habitat and other components of the ecosystem and their interactions (Browman and Stergiou, 2004).
Such an integrated multispecies approach to management measure is particularly essential in the multispecies Mediterranean fishery context, where the large number of species caught makes calculations for a single species of limited value for management (Caddy, 1990, 1993).
1.2. The study area
The Balearic Islands (western Mediterranean, Figure 1.2) are sited in the Balearic Promontory, a structural elevation 348 km in length, 105 km wide and from 1000 to 2000 m high with respect to the surrounding basins. It is 95 nautical miles away from the Iberian Peninsula, separated by depths of between 800 and 2000 m. The Balearic Promontory delimits the Balearic sub-basin (BsB, in the north) from the Algerian sub-basin (AsB, in the south). The shelf in the Balearic archipelago is narrow
36
and steep on the northern side and wider with a gentler slope in the south. In the southeast of Mallorca there are numerous small canyons. The biggest canyon in the Balearic Islands is situated in the south of Menorca (Figure 1.2; Acosta et al., 2004).
There is not much terrigenous-muddy sediment due to the absence of river discharges.
Sandy-muddy and detrital sediments occur at the shelf-slope break, whereas muddy bottoms of biogenic origin dominate the deeper areas (Acosta et al., 2002).
Figure 1.2. Map of the study area, showing the 200, 600, 800, 1000 and 2000 m isobaths. Mc:
Menorca canyon.
The hydrographic conditions of the Balearic Islands have been extensively studied and the Balearic channels have been shown to control the regional circulation around the Balearic Islands (Pinot et al., 2002). The regional circulation in the area is
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
38 38.5 39 39.5 40 40.5 41 41.5 42
MALLORCA
MENORCA
Mc
Longitude
L a ti tude
Balearic Islands
-6 -4 -2 0 2 4 6
35 37 39 41 43
0º 0.5ºE 1ºE 1.5ºE 2ºE 2.5ºE 3ºE 3.5ºE 4ºE 4.5ºE 5ºE
6ºW 4ºW 2ºW 0º 2ºE 4ºE 6ºE
41ºN
40.5ºN
40ºN
39.5ºN
39ºN
38.5ºN
38ºN
43ºN
41ºN
39ºN
37ºN
35ºN
IBIZA
FORMENTERA
37
dominated by the Northern Current (NC) which carries down Atlantic waters (AW) from the Gulf of Lions along the continental slope of the Iberian Peninsula into the BsB (Font et al., 1988). This current bifurcates when reaching the Ibiza Channel; one significant part crosses the channel transporting waters from the Mediterranean into the AsB, while other part cyclonically returns to the northeast forming the Balearic Current (BC) along the northern coasts of the Balearic Islands. This latter current is also increased by flows of AW passing northwards through both channels (Figure 1.3.a).
Figure 1.3. Averaged regional ocean circulation in the western Mediterranean corresponding to late spring–summer after a relatively mild winter, May 1997 (a) and after a relatively cold winter, May 1996 (b). Major currents in the region, the Northern Current (NC) and the Balearic Current (BC) are indicated. From Monserrat et al. (2008). AW: Atlantic Water; WIW: Western Mediterranean Intermediate Water; LIW: Levantine Intermediate Water.
This smooth general pattern is the one expected for late spring–summer after a previous relatively mild winter in the western Mediterranean, but fluctuates considerably in mesoscale, seasonal and interannual time scales, changing dramatically after a cold winter (García Lafuente et al., 1995; Pinot et al., 2002). The NC may be blocked when reaching the Ibiza Channel and then recirculates cyclonically joining the BC without significant transport of waters through the Ibiza Channel (Figure 1.3.b).
This blocking has been related to the recurrent existence of anticyclonic channel size eddies trapped at the coastal edge of the NC in the Gulf of Valencia (Castellón et al., 1990; Pinot et al., 1995; Pinot and Ganachaud, 1999) and more precisely to the presence of significant amount of subsurface waters of Mediterranean origin, characterized by low temperatures (less than 13 °C) forming the anticyclonic gyres (Pinot et al., 2002).
This cold and relatively fresh waters, known as Western Mediterranean Intermediate Waters (WIW; Table 1.1) are generated during winter in the Gulf of Lions by deep
38
convection when sea–air heat flux losses are high enough (Mertens and Schott, 1998;
Pinot et al., 2002). The Gulf of Lions, where deep convection usually takes place, is subject in winter to cold and dry winds from north and northwest, usually occurring in strong bursts and contributing to increase both sensible and latent heat flux losses from the ocean to the atmosphere. When this forcing is strong enough, during winter, weakly stratified underwaters, including salty Levantine Intermediate Waters (LIW, Table 1.1), may become exposed to the surface buoyancy flux and deep convection can occur to intermediate or full ocean depths (Millot, 1987; Schott et al., 1996; Mertens and Schott, 1998). Then, these dense waters spread into the western basin, mainly as a boundary current along the continental slope (Send et al., 1996).
Table 1.1. Characteristic values of potential temperature (θ; ºC) and salinity (S; psu) of the different water types and local values at the Balearic Islands (López-Jurado et al., 2008). AW:
Atlantic Water; WIW: Western Mediterranean Intermediate Water; LIW: Levantine Intermediate Water; WMDW: Western Mediterranean Deep Water.
When the WIW, normally located between 100 and 300 m, are present in the Balearic Channels, they usually deflect downwards the more saline LIW which normally occupies these levels when WIW are absent. This fact is clearly seen both in temperature and salinity cross sections when comparing the situation after a mild winter with that observed after a colder one (Figure 1.4). The observed interannual variability of the regional circulation around the Balearic Islands is strongly related to the properties and the amount of WIW reaching the channels in late spring (Millot, 1999;
Pinot et al., 2002). This interannual variability modifies in turn the hydrographical properties of the waters surrounding the islands.
Water mass Values at origin Local values AW 15.0 < θ < 18.0 15.0 < θ < 28.0
36.15 < S < 36.50 36.50 < S < 37.50 WIW 12.5 < θ <13.0 12.5 < θ < 13.0
37.90 < S < 38.30 37.90 < S < 38.30 LIW 14.0 < θ < 15.0 13.0 < θ < 13.4
38.70 < S < 38.80 38.45 < S < 38.60 WMDW 12.7 < θ < 12.9 12.7 < θ < 12.9
38.40 < S < 38.48 38.40 < S < 38.48
39
Figure 1.4. Vertical cross sections of potential temperature (°C) and salinity along the Ibiza channel after a relatively mild winter, May 1997 (a, b) and after a relatively cold winter, May 1996 (c, d). From Monserrat et al. (2008), after López-Jurado et al. (2001).
40
The Balearic channels have also been described as important passages for the exchange between the cooler, more saline waters of the BsB and the warmer, fresher waters of the AsB (Pinot et al., 2002). These marked differences between the north and south are reflected in the structure of the water column. Western Mediterranean Deep Waters (WMDW, Table 1.1) appear deeper in the south, which implies that the LIW layer is wider as well as warmer and more saline. Both temperature and salinity show more variability in the north than in the south, where the water column variations seem to be much smoother, as the south is less affected by the above mentioned processes (López-Jurado et al., 2008).
Within the general oligotrophic environment of the Mediterranean (Figure 1.5), the waters around the Balearic archipelago, where there is no supply of nutrients from land runoff, show more pronounced oligotrophy than the adjacent waters off the Iberian coast and the Gulf of Lions (Estrada, 1996; Bosc et al., 2004). Frontal meso-scale events between Mediterranean waters and waters of Atlantic origin (Pinot et al., 1995) and input of cold northern water into the channels (Fernández de Puelles et al., 2004) act as external fertilisation mechanisms that enhance productivity off the Balearic Islands. Trophic web structures show differences between the BsB and AsB, as the trophic webs are supported more by plankton biomass than by benthos in the AsB, while suprabenthos plays a more important role in the BsB (Maynou and Cartes, 2000;
Cartes et al., 2001, 2008a).
Figure 1.5. Monthly map (May 1999) of the chlorophyll concentrations. From Bosc et al.
(2004).
41
1.3. The bottom trawl fishery off the Balearic Islands
Although it is not clear when the trawl fishery started in the Balearic Islands, the most probably date is during the last quarter of the 18th Century (Darder and Oliver, 2007). The exploitation of fishing grounds between 100 and 300 m depth started in 1939, due to the first signals of depletion of shallower grounds (Massutí, 1959, 1973).
Deeper fishing grounds located between 300 and 800 m depth, which were formerly unknown or impracticable, started to be exploited in 1948, coinciding with its charting by the Instituto Español de Oceanografía (IEO) (Massutí, 1959, 1963; Massutí and Oliver, 1975).
The total number of trawlers operating off the Balearic Islands has decreased from the seventies but the mean horsepower has increased steadily since the beginning.
The trawlers operating in Mallorca reached its maximum in 1977 (70) and started declining since then, until the current number (32) in 2010 (Figure 1.6). However, the real mean horsepower (which has been estimated to be 2.4 times higher than the nominal horsepower) by year has been increasing between 1965 and 2010 (Figure 1.6).
Figure 1.6. Number of trawlers (continuous line) and real mean horsepower (HP) per trawler (discontinuous line) by year for the bottom trawl fleet of Mallorca. From Quetglas et al. (2009).
Year
1968 1973 1978 1983 1988 1993 1998 2003 2008
Number of trawlers
0 10 20 30 40 50 60 70 80
Mean HP
0 100 200 300 400 500 600 700
42
In the Balearic Islands, bottom trawlers operate along a wide bathymetric range, from the shallow shelf (deeper than 50 m depth) to the middle slope (down to 800 m).
They are multispecific and apply different fishing tactics (FT) even during the same fishing trip, depending on the season, weather conditions, and commercial factors (e.g., landing price, day of the week or boat order in the auction).
Up to four individual fishing tactics can be distinguished, being the most important species and depth range exploited the following (Moranta et al., 2008a;
Palmer et al., 2009):
- Shallow shelf (SS; 40-80 m): targeting Spicara smaris, Mullus surmuletus, Octopus vulgaris and catching other species such as Trachurus mediterraneus, Scyliorhinus canicula, Loligo vulgaris, and a mixed fish category called “morralla”, composed by Trachinus draco, Serranus cabrilla, Chelidonichthys lastoviza, Pagellus spp., Scorpaena notata, Scorpaena scrofa, among others;
- Deep shelf (DS; 125-235 m): targeting Merluccius merluccius and catching other species such as Trachurus trachurus, Illex coindetti, Raja spp., S. canicula, and a mixed fish category also called “morralla” but with a different composition from the SS
“morralla”: Chelidonichthys cuculus, T. draco, Trigla lyra, S. cabrilla, Trisopterus minutus, Helicolenus dactylopterus, among others.
- Upper slope (US; 350-600 m): targeting Nephrops norvegicus but with an important by-catch of Micromesistius poutassou, M. merluccius, I. coindetti, Parapenaeus longirostris, Lepidorhombus boscii, Lophius piscatorius, H. dactylopterus and Phycis blennoides.
- Middle slope (MS; 600-750 m): the only target species is Aristeus antennatus, which is taken along with a by-catch that includes Galeus melastomus, Geryon longipes, M.
merluccius, M. poutassou and P. blennoides.
Although catches in the daily sale bills for each boat reflect a highly diverse mixture of species, these catches taken on such different fishing grounds can be identified from daily sale bills, because these fishing grounds have distinctive bottoms that are characterized by different assemblages of demersal species (Massutí and Reñones, 2005; Moranta et al., 2008a). Thus, the analysis of discriminant analysis and artificial neural networks can allow us to estimate which FTs have been applied in each
43
fishing trip (see Chapter 2). For a simpler interpretation, for those fishing trips in which two different FTs were performed, values on biomass and economic value were equally distributed for each FT and each fishing trip was computed as half day for each FT.
The most important fishing tactics in terms of total biomass landed by the bottom trawl fleet from Mallorca are those developed in the shelf, which represent up to 57% (40% SS and 17% DS, Figure 1.7), in front of the slope tactics, which represent 43% (19% US, 24% MS). In terms of fishing effort, the most important fishing tactic is the MS, which represents nearly half of the fishing trips (49%), followed by SS (26%), US (14%) and DS (11%). The same proportions are obtained when considering the economic value of the biomass landed.
Figure 1.7. Percentage of landed biomass, fishing effort (number of trips) and economic value for each of the fishing tactics performed by the bottom trawl fleet from Mallorca, between 2000 and 2009. SS: shallow shelf; DS: deep shelf; US: upper slope; MS: middle slope.
The performance of effort targeting each fishing tactic varies among the different ports of Mallorca (Figure 1.8). For those ports whose fleet operates in the fishing grounds sited in the south and west of Mallorca (Andratx, Palma and Sóller) and one in the north (Pollença), the most important fishing tactic in terms of fishing effort is the MS (54-73%), while the rest of fishing tactics represent between 5-20%. For those ports whose fleet operates in the fishing grounds sited in the east and northeast of Mallorca (Portocolom and Cala Rajada), the most important fishing tactic is the SS, which represents around half of the fishing days (49-54%) but being the MS also very important (30-40%). There are two ports whose characteristics do not fit which any of these previous two groups: Alcúdia (north of Mallorca), in which MS is very important but followed by DS (30%) and Santanyí (southeast of Mallorca) in which most of the fishing days correspond to fishing tactics from the shelf (SS 82% and DS 15%).
44
Figure 1.8. Percentage of fishing effort (number of trips) by fishing tactic for the bottom trawl fleet from Mallorca, for each port, between 2000 and 2009. SS: shallow shelf; DS: deep shelf;
US: upper slope; MS: middle slope. The map shows the spatial distribution of the ports, showing the 200 and 800 m depth isobaths. PO: Pollença; AL: Alcúdia; CR: Cala Rajada; PC:
Portocolom; SA: Santanyí; PM: Palma; AN: Andratx; SO: Sóller.
When computing the total number of fishing days by fishing tactic and year, there is a clear decreasing trend for all the fishing tactics, except in the case of DS, in which it remains more or less stable (Figure 1.9). This trend can be related to the above mentioned decrease of the number of trawlers during last 10 years. By month, there is a clear increase during summer months in the MS, with only a small decrease in SS and DS during this season, while for US remain practically constant during all year.
2.5ºE 3ºE 3.5ºE
39ºN 39.5ºN 40ºN
PM AL PO
AN
SO CR
PC SA
45
Figure 1.9. Total number of fishing trips by year (up) and month (down) for each of the fishing tactics performed by the bottom trawl fleet from Mallorca, between 2000 and 2009. SS: shallow shelf; DS: deep shelf; US: upper slope; MS: middle slope.
1.4. The deep water crustacean fishery
The knowledge and exploitation of deep water fishing grounds resulted in the apparition of new commercial species, especially crustaceans which acquired the highest economic interest. Later, the increase in the knowledge of new fishing grounds and the improvement and modernization of the fishing boats resulted in an extraordinary increase of crustacean catches. Under the denomination “shrimp”, several species were included: the red shrimp A. antennatus, the giant red shrimp Aristaeomorpha foliacea, the deep water rose shrimp P. longirostris and the genus Plesionika (Oliver Massutí, 1953). The catch curve of these shrimps (Figure 1.10) went up enormously during the first years, and reached two maximum values in 1958 and
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1990 (300-350 t), with important oscillations between them. After this second maximum, landings have shown a slight decreasing trend.
Figure 1.10. Landings of shrimps (A. antennatus, A. foliacea, P. longirostris and Plesionika spp.) from the bottom trawl fleet from Mallorca (Balearic Islands).
The first deep water fishing grounds exploited (i.e., fishing grounds from the slope, found deeper than 200 m, and thus corresponding to the US and MS fishing tactics) were those sited in the south of Mallorca, going the exploitation northern until reaching the fishing grounds of Menorca (Figure 1.11; Oliver, 1983). Nowadays, the only impracticable grounds in the slope are sited in the southeast of Mallorca and northeast of Menorca (Figure 1.12).
Figure 1.11. Evolution of the deep water fishing grounds exploited by the bottom trawl fleets from Mallorca and Menorca. From Massutí (1971) and Oliver (1983).
Year
1948 1953 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008
Landings (t)
0 100 200 300 400
